A tip, if you allow me: when you go to Edit > Colors, and click on New button, notice a dropdown list in upper right corner which contains a lot of color sets. Using, e.g., "Scribus X11 Grey" might be helpful as starting point for creating your desired grey (even if creating grey shouldn't be hard: set C, M and Y values to zero, and change value of K only; but for creating other colors this feature might be of great help).

Note: I assume you're using Scribus 1.4.0. I'm not sure about 1.3.3.x series.

...BTW, for what is transparaency, when the usage of transparency is seen as an error?

Since I'm already here, I hope a.l.e wouldn't mind if I jump in

It's more of a warning, actually, depending on format and purpose of your document. Short version: if you intend to create material for print and/or have to use PDF/X-3, you'll get this warning so that you're aware that PDF/X-3 does not support transparency and your document will probably not look as expected (in other words: expect problems). But, if you use, e.g., PDF 1.5 and/or create material for online reading, this warning will disappear, since PDF 1.5 does support transparency. However, keep in mind that PDF 1.5 probably isn't what your printer would like to see.

Note: "Background" in "object has transparency background" is actually name of the layer.

...BTW, for what is transparaency, when the usage of transparency is seen as an error?

Since I'm already here, I hope a.l.e wouldn't mind if I jump in

It's more of a warning, actually, depending on format and purpose of your document. Short version: if you intend to create material for print and/or have to use PDF/X-3, you'll get this warning so that you're aware that PDF/X-3 does not support transparency and your document will probably not look as expected (in other words: expect problems). But, if you use, e.g., PDF 1.5 and/or create material for online reading, this warning will disappear, since PDF 1.5 does support transparency. However, keep in mind that PDF 1.5 probably isn't what your printer would like to see.

Note: "Background" in "object has transparency background" is actually name of the layer.

Thanks! Just studied Wikipedia article about PDF X/3 and I understand now the problem